Secondary hydrothermal minerals in buried rocks at the ...periodicodimineralogia.it/2011_80_3/2011PM0027.pdfSecondary hydrothermal minerals in buried rocks at the Campi Flegrei caldera,

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYand applied topics on Environment, Archeometry and Cultural Heritage

DOI: 10.2451/2011PM0027Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 385-406

PERIODICO di MINERALOGIAestablished in 1930

Secondary hydrothermal minerals in buried rocksat the Campi Flegrei caldera, Italy: a possible tool to understandthe rock-physics and to assess the state of the volcanic system

Angela Mormone1,2,*, Anna Tramelli1, Mauro A. Di Vito1, Monica Piochi1, Claudia Troise1

and Giuseppe De Natale1

1 Istituto Nazionale di Geofisica e Vulcanologia, sezione Osservatorio Vesuviano, Napoli, Italy2 Dipartimento di Scienze della Terra, Università degli Studi di Napoli Federico II,

Via Mezzocannone 8, 80134 Napoli, Italy*Corresponding author: [email protected]

Abstract

The distribution of the alteration assemblages and the related physico-chemical changesinduced in the rocks with depth, may provide useful information on the state of the system.Drillholes are the only way to define hydrothermal alteration depth-profiles in variablegeological contexts. Deep drillings exploiting programs were conducted since the 1970’s bythe Agip-Enel Joint Venture in the Quaternary Campi Flegrei caldera (southern Italy), where ageothermal system has been active since at least historical times. New macroscopic andmicroscopic investigations were performed on selected samples made available by Agip inorder to: 1) define the precursor lithology, 2) describe the relationships among texture,mineralogy and depth of the studied core samples and 3) examine the character of the secondaryminerals and their distribution with depth and temperature. The new data are integrated withphysical properties and elastic parameters of cored rocks, as well as structural information andfield data, all available from the physical, seismological, geodetical and volcanological literature.The depth-related multi-parameters profiles provide evidence on the different behavior of theburied rocks beneath the Licola 1, Mofete and San Vito 1 areas, sited in three structurallydifferent sectors of the caldera. The features of the hydrothermally altered rocks are a key tointerpret the heterogeneities of the Campi Flegrei substratum, as deduced by velocity, attenuationand scattering P- and S- waves tomography. The time and space distribution of both the eruptivevents and the extruded magma volumes are consistent with the results of our analysis. Therefore,we interpret the observed Campi Flegrei geothermal system as a response to the distribution ofvolcanic activity in two structurally distinct sectors of the caldera. The central-eastern sector,where the San Vito 1 well was drilled, represents the preferential pathways for both gas escapeand magma ascent at least since 8 kyrs, in contrast with the other sites of the caldera whereeruptions occurred with minor frequency and magnitude.

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386 A. Mormone et al.Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 385-406

Introduction

Secondary (hydrothermal) mineral phaseschange the primary mineral assemblages andchemistry of rocks (e.g., Browne, 1978), as wellas their physical features, and, in particular, theirelastic behaviour (e.g., Zamora et al., 1994). Inactive volcanic areas, high-temperature, salineand deep-sourced fluids normally inducedevitrification of volcanic glass, mineral phasesdissolution and re-crystallization of new mineralphases in buried deposits (Browne, 1978). Themain factors affecting these processes are i)fluids temperature and chemistry, and ii) theoriginal rock type, porosity and permeability, inaddition to the time interval of fluid circulation.Notably, all these factors strictly depend on thearchitecture of the volcanic system, especiallythe structural setting which determines the fluidspathways. Therefore, knowledge of the depthdistribution of hydrothermal mineral phases,together with data on aquifer location andchemistry, is an important topic in studying avolcanic area and the behaviour of its associatedgeothermal system, constraining geophysicalinvestigations, and, ultimately, providinginformation for the possible exploitation ofgeothermal energy and/or mineral systems(Browne and Ellis, 1970; Giggenbach, 1991;Pirajno, 1992; Barbier, 2002).

Areas of geothermal activity are common onEarth. In Italy, they are mostly associated tovolcanic systems located along a NW-trendingbelt (Figure 1a) (Bertini et al., 2005). From Northto South, we mention, for example, theLarderello (e.g., Batini et al., 2003) and theLatera caldera (e.g., Cavarretta et al., 1982) inTuscany, the Campi Flegrei caldera (e.g.,Guglielmetti, 1986; Rosi and Sbrana, 1987) and

Ischia island (e.g., Sbrana et al., 2009) inCampania, Vulcano (e.g., Aubert et al., 2007) andPantelleria (e.g., Gianelli and Grassi, 2001)islands, in Sicily. In particular, the active CampiFlegrei caldera (Figure 1b) hosts a geothermalsystem, which has been investigated for morethat 40 years by drilling (Rosi et al., 1983;Guglielmetti, 1986; Agip, 1987; Rosi and Sbrana,1987; De Vivo et al., 1989). Deep wellsintersected the shallow crust (Figure 2) reachingdepths ranging from 1600 m and 3000 m b.s.l.and showing a particularly interesting andcomplex geological setting. Notably, theyevidenced the presence of high geothermalgradient and aquifers with temperature of > 350°C and salinity of up to 20 wt% at depths of about2700 m (Guglielemetti, 1986). Cored sampleswere the object of diverse studies (e.g., Agip,1987; Rosi and Sbrana, 1987; De Vivo et al.,1989; Zamora et al., 1994; Caprarelli et al., 1997;Vanorio et al., 2002; Giberti et al., 2006; Vanorioet al., 2005) offering an accurate and detailed,albeit inhomogeneous and discontinuous,representation of the rock types and structurebeneath the Campi Flegrei caldera, providing apowerful database for current and futureresearch. In particular, Rosi et al. (1983), Rosiand Sbrana (1987), De Vivo et al. (1989) andCaprarelli et al. (1997) published on thefollowing: i) the litho-stratigraphy, ii) the thermalgradient, iii) the distribution with depth of themain alteration mineral assemblages, and iv) thefossil temperature and salinity of fluid inclusions.Zamora et al. (1994), Vanorio et al. (2002) andVanorio et al. (2005) determined some of thephysical properties of core samples, mainlydensity and porosity and, in some casespermeability and P- and S-waves velocity.However, most recent literature focused on the

Key words: Campi Flegrei; elastic proprieties; geothermal system; hydrothermal minerals;volcanism.


reconstruction of the history of the volcano andof its magmatic feeding system (e.g., Arienzo etal., 2010; Vilardo et al., 2010; Di Renzo et al.,2011; Mormone et al., 2011; and references

therein). Other authors investigated the caldera’sactive geothermal system focussing on watertables and/or fumaroles emissions characteristics(Valentino et al., 1999; Chiodini et al., 2001;

387Secondary hydrothermal mineralsin buried rock...Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 385-406

Figure 1. a) Geological sketch map of Italy showing the location of the main geothermal fields (modified fromCarella et al., 1985 and Bertini et al., 2005), the CO2 emission area (modified from Frezzotti et al., 2009), thedistribution of the heat flux (mW/m2; modified from Della Vedova et al., 1991), main tectonic trends and theCampi Flegrei site (modified from Acocella et al., 1999). b) Structural map of the Campi Flegrei caldera withthe eruptive vent locations in the last 14.9 ka (from Di Renzo et al., 2011), distribution of the temperature > 100°C at 1000 m b.s.l. (modified from Corrado et al., 1998), area of stability or ground movements (from Di Renzoet al., 2011) and the location of Agip’s (1987) boreholes.


A. Mormone et al.388 Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 385-406

Figure 2. Representative well stratigraphy at the Mofete and San Vito 1 sites (from Rosi et al., 1983).



Valentino and Stanzione, 2004) or thermal andflux simulation modelling (Gaeta et al., 1998; DeLorenzo et al., 2001; Troiano et al., 2011 andreferences therein). Geophysical investigationsallowed information on the main crustal structure(Zollo et al., 2008; Trasatti et al., 2011) throughthe use of velocity (Aster et al., 1989; Judenhercand Zollo, 2005; Battaglia et al., 2008),attenuation (De Lorenzo et al., 2007; De Sienaet al., 2010) and scattering (Tramelli et al., 2006)seismic tomography.

This paper presents the results obtained fromoptical and electron microscopy on 22 coresamples, drilled by Agip for geothermal energyresearch (AGIP, 1987). Our results were firstlyintegrated with those of the previously citedliterature in order to augment our knowledge onthe secondary mineralogy and its relationshipwith the original rock textures and depthdistribution. Then, the integration of litho-stratigraphic, hydrothermal alteration distributionand temperature data with physical parametersand P- and S- wave velocities and Vp/Vs ratiosby selected physical and seismological studies(Zamora et al., 1994; Tramelli et al., 2006;Battaglia et al., 2008; De Siena et al., 2010)allowed the reproduction of multi-parametersdepth-related profiles (Figure 3). Finally,volcanological and structural data (Di Vito et al.,1999; Orsi et al., 2009; Vilardo et al., 2010; DiRenzo et al., 2011) and, in particular, the ventslocation and the volume of erupted magmas atCampi Flegrei in the past 14.9 ka, allow topresent a model corroborating the idea that thecaldera consists of two distinct sectors (Figure 4).

Geological and geophysical backgrounds

The Campi Flegrei caldera is a ~ 10-kmdiameter collapsed area. Its volcanism is olderthan 60 ka (Pappalardo et al., 1999) and calderacollapses occurred during two catastrophiceruptions at 39 ka (De Vivo et al., 2001) and 14.9ka (Deino et al., 2004). At least 73 explosive

eruptions took place within the caldera after thesecond collapse (Orsi et al., 2009; Di Renzo etal., 2011 and references therein). The lasteruption formed the Monte Nuovo cone in 1538A.D. (Rosi and Sbrana, 1987; Di Vito et al.,1999). The Campi Flegrei’s rocks are generallytrachytic to trachy-phonolitic in composition andmineralogically dominated by alkali (mostlypotassic)-feldspars, plagioclase, Fe-oxides,apatite, diopside and biotite (e.g., Pappalardo etal., 2002 and references therein). Olivinephenocrysts are not common, leucite and sodaliteare sporadically present and quartz is only foundas xenocryst (Rosi and Sbrana, 1987). Least-evolved magmas were occasionally eruptedduring low-magnitude explosive events alongregional faults (Di Renzo et al., 2011 andreferences therein). Chemical and mineralogicalfeatures of volcanic rocks derived from polybaricevolution processes in magmas stored within thecrust (e.g., Pappalardo et al., 2002; Di Renzo etal., 2011; Mormone et al., 2011).

Based on the most recent research, the CampiFlegrei magmatic system consists of a deeperreservoir identified at about 7.5 km from largeamplitude seismic reflection at the top of anextended melt-bearing rock formation (Zollo etal., 2008). Notably, this system coincides withthe ~ 350 MPa entrapment pressures of meltinclusions within the host-olivine from the 18 kaSolchiaro 1 eruption products (Mormone et al.,2011). This suggests a long-lived deep magmareservoir beneath the Gulf of Pozzuoli (seeFigure 1b). The chemical features of the melt-inclusions indicate a trachybasaltic compositionand CO2-dominated gaseous phases for themagma in this reservoir (Mormone et al., 2011).At shallower depth, less than 3 km, seismic datado not evidence magmatic bodies larger than 1km3 (Zollo et al., 2008). This contrasts withresults from melt inclusions studies suggestingthat magma storage frequently occurred in thepressure range 150-to-40 MPa before severalexplosive eruptions of the last 39 ka (Cecchetti



Figure 3. New data on the main hydrothermal mineral assemblage defined and observed (Figure 5) in rockscored at the Mofete 1, Mofete 5, Vito 1 and Licola 1 wells. The last five column diagrams on the right showprofiles of temperature, density, porosity, Vp, Vp/Vs as derived by literature (De Vivo et al., 1989; Zamora etal., 1994; Vanorio et al., 2002; Vanorio et al., 2005; Battaglia et al., 2008) data. For comparison and completeness,these columns also report data for San Vito 3 and Mofete 2 wells (open black triangles and open gray rhombus,respectively). Note: squares are Vp and Vp/Vs from active seismology; the yellow box indicates data onoutcropping rocks, the pink boxes include elastic parameters measured on the outcropping rocks (tuffs and lavas,dark and bright pink colour, respectively) at room temperature and under indicate pressure (i.e., depth).



et al., 2003; Marianelli et al., 2006; Arienzo etal., 2010; Mormone et al., 2011). Besides,petrological studies (Di Renzo et al., 2011)corroborate the hypothesis that the CampiFlegrei magmatic system is formed by tworeservoirs, the shallower of which, located at 6-4 km of depth, is discontinuous andcharacterized by small trachy-phonolitic magmapockets.

The deeper system is likely hosted by aHercynian-type crust on the basis of xenolithsrecovered in the least evolved magmas(Pappalardo et al., 2002); whereas the uppermostcrust is not well-defined. In particular, theMiocene limestone and sandstone sequences thatshould be present as they constitute the nearbyApennine Chain and are revealed by seismicinvestigations in the majority of the CampanianPlain (Piochi et al., 2005 and references therein),have been not recovered beneath the caldera.Limestone xenoliths are not reported in thecurrent literature. The stratigraphy of theuppermost 3000 meters has been directly definedthanks to drilling programs by Agip (1987) andsynthesized in various papers (Rosi et al., 1983;Guglielmetti, 1986; Rosi and Sbrana, 1987;Caprarelli et al., 1997). We use the mostrepresentative one (Figure 2) from Rosi et al.(1983). These authors suggest variouspumiceous, tufaceous, marine tuffites and lavaflows overlying thermally metamorphosed rocksat > 2500 m b.s.l. Generally, thin furthersandstone levels occur deep in the holes.According to the Authors, the sequence drilled inthe San Vito plain mostly resulted by calderafilling phenomena. Furthermore, the thermalmetamorphism was interpreted as due to theproximity of hot magma batches. Moreover, incontrast with the Mofete wells, for the San Vito1 well Rosi et al. (1983) also show the presenceof several intrusions deeper than 1500 (Figure 2).

Present fumaroles and thermal springs are themain evidence of the hydrothermalism that likelyoriginated the well-developed alteration mineral

zoning (Rosi et al., 1983; Rosi and Sbrana, 1987;Caprarelli et al., 1997; Guglielmetti, 1986).Fumaroles concentrate within and nearby theSolfatara crater where they are associated to atleast 1500 tons/day of CO2 emissions (Chiodiniet al., 2001). Thermal springs are widespread inthe caldera and are linked to shallow and locallydiscrete aquifers. The drilling programs carriedout by Agip (1987) showed the presence of atleast three main aquifers in the Mofete area(Guglielmetti, 1986), with the deepest at about2300 m, showing the highest salinity. Thechemical composition of thermal waters andfumaroles suggest that meteoric and sea waterfrom the nearby Gulf of Pozzuoli (e.g., Cortecciet al., 1978; De Vivo et al., 1989) are the mainsources of circulating fluids. Magmatic gasescontribute to the composition of shallower fluids(e.g., Baldi et al., 1975; Bolognesi et al., 1986),also supply most of the carbon dioxide (e.g.,Caliro et al., 2007) and are suspected to be themain engine for uplifting processes episodes(Todesco et al., 2010; Troiano et al., 2011).Ground deformation occurring continuously sinceRoman times is characterized by subsidence anduplift affecting the entire caldera depression. Themain uplift episode preceded the Monte Nuovoeruption, whereas minor and intense episodes alsooccurred in 1969-72 and 1982-1984, determininga net vertical ground deformation along thePozzuoli coast of 3.5 m. More than 16,000earthquakes occurred in the Solfatara area duringthe 1982-84 unrest episode (Barberi et al., 1984;Aster et al., 1989; D’Auria et al., 2011).

Volcanological and structural dataThe structural setting of the Campi Flegrei is

the result of complex interactions betweenregional tectonics and volcanism (Figure 1b).Regional faults have strike mainly NW-SE andNE-SW (Rosi and Sbrana, 1987; Orsi et al.,1996). Faults and eruptive fissures within theyoungest caldera show variable orientations (DiVito et al., 1999; Vilardo et al., 2010; Di Vito et


al., 2011): i) NW-SE and NE-SW trends on thecentral-eastern side, and, ii) NE-SW and N-S inthe central-western side. Satellite images(Vilardo et al., 2010) show extensivedeformation within the caldera and evidence thatthe maximum horizontal strain affects its central-eastern sector, where recent volcanism (3.8-4ka), active degassing and seismicity areconcentrated. These authors also note in thePozzuoli centre, the presence of a high P-waveseismic attenuation and S-wave polarizationoriented following the N-S direction (see Figure1 in Vilardo et al., 2010).

We strongly support the Vilardo et al.’s modeland, additionally, we highlight that 76% of thevents in the past 15 ka opened in the central-eastern sector of the caldera (Figure 1b). We alsoevidence that in the past 15 Ka about 4 km3 ofmagma has been extruded in the central-easternsector, whereas only 1 km3 on the opposite side(Figure 4). Notably, the western sector wascharacterized by highly-evolved trachytic totrachyphonolitic magmas and low-intenseexplosive, phreato-magmatic eruptions.Conversely, the eastern sector was the site ofplinian to strombolian, magmatic tophreatomagmatic events that produced rockswith variable evolution degree, from shoshoniteto trachyphonolite.

On the basis of the above, Agip’s wells weredrilled in three different structural locations(Figure 1): 1) the Licola 1 well is external to thecaldera depression, located nearby its north-western margin (Orsi et al., 1996), 2) the Mofeteand San Vito 3 wells were drilled in theproximity of the structural margin of theNeapolitan Yellow Tuff caldera and 3) the SanVito 1 well is clearly within the caldera. Inparticular, the Mofete 1, 2, 5 wells are along thewestern caldera margin, dominated by N-Strending fault system; the San Vito 3 is inproximity of the NNW caldera boundary, wherelow-evolved magmas were erupted, likely alongregional faults. Whereas, the San Vito 1 is in the

eastern sector, characterized by recent intenseand frequent volcanism. It is also the mostproximal to the city of Pozzuoli, subject to themaximum uplift both during the lastbradyseismic events (Orsi et al., 1999) and thelong-term ground deformation of the calderafloor (Di Vito et al., 1999).

Seismological dataThe knowledge of the very complex

subsurface structure of the Phlegraean calderaderives from several geophysical analyses thatwere performed in this area. We are mainlyinterested on seismic wave velocities (Aster etal., 1989; Judenherc and Zollo, 2005; Battagliaet al., 2008), attenuation (De Lorenzo et al.,2001; De Siena et al., 2010) and scattering(Tramelli et al., 2006) tomographies that provideinformation about the elastic properties of themedium and are strictly related to the physicalproperties of the rocks. Most of the seismicinvestigations used the high number of seismictraces recorded during the 1982-1984bradiseismic event. De Siena et al. (2010)analyzed these seismograms to define theattenuation properties of the travelled medium.Temperature, presence of fractures and type offluid filling rock voids affect the seismic waveattenuation, calculated from the ratio of the peakstrain energy stored in a volume, respect to theenergy lost in each cycle. Tramelli et al. (2006)analyzed the codas of the same seismograms todefine the scattering properties of the medium.These properties principally depend on the rockheterogeneities (mainly, fracture), the size ofwhich can be determined from the frequencyanalysis in the coda waves. The scatteringtomography is expressed in terms of relativescattering coefficient that can be interpreted asthe mean free path of the seismic ray, i.e., themean length that the seismic ray can travelbefore intercepting a scatterer. Finally, Battagliaet al. (2008) merged the passive seismic signalsderived from > 15000 micro-earthquakes with



Secondary hydrothermal mineralsin buried rock... 393Periodico di Mineralogia (2011), 80, 3 (Spec. Issue), 385-406

Figure 4. Volume (km3 DRE) of products erupted through time in the past 14.9 ka in the two structurally anddynamically different sectors of the Campi Flegrei caldera. Shaded areas envelop the epochs of the volcanism,following Di Vito et al. (1999): I epoch between < 14.9 and 9.5 ka, III epoch between 8.6 and 8.2 ka, and IIIepoch between 4.8 and 3.8 ka, as indicated on the diagrams. Arrows indicate the highest magnitude and the lasteruptions of Agnano Monte Spina and Monte Nuovo, respectively. See text for further details.


A. Mormone et al.394

those from active seismology. These lattersignals were produced by the 1528 shots of the2001-SERAPIS experiment. The authorsdescribed the Vp and Vp/Vs ratio thatcharacterize the medium in a volume of 18 x 18x 9 km starting from 0.5 km a.s.l., with a nodespacing of 0.5 km. Interestingly, the Vp/Vsvalues were long recognized as lithologicalindicators (Wang, 2000 and references therein).The results obtained by Battaglia et al. (2008)are represented in Figures 3 and 6.

An arc-like positive Vp values anomalydefines the southern caldera border, offshore,from Capo Miseno to Nisida (Battaglia et al.,2008). The measured Vp of 3.5-4 km/s is higherthan the values of 2.5-3 km/s typically of thecaldera filling sediments present at the centre ofthe Gulf. Here, a shallow (1-2 km depth) layerwith high Vp/Vs ratio (2.3-2.7) overlies an areaof anomalously low Vp/Vs ratio (1.2-1.4),between 3 and 4 km b.s.l. This low Vp/Vsvolume almost attenuates P and S waves (DeSiena et al., 2010) and, based on the distributionof the high seismic waves attenuation, likelyextends in the Pozzuoli-Solfatara area. A furtherhigh attenuation rock volume, for both P and Swaves, occurs beneath Pozzuoli, between 0 and-3 km, beneath the area between Mofete andMonte Nuovo, between -0.5 km and -2.5 km, andbeneath Agnano, between -1 km and -3 km. Themedium seems to be more homogeneous below-3 km. The on land caldera border is a highlyheterogeneous volume, clearly identified by thescattering tomography (Tramelli et al., 2006).Scattering volumes also occur below theSolfatara crater and offshore of Baia. Based onthe frequencies of scattering, the Authors suggestthat the two volumes characterize for thefractures size, being larger offshore of Baia. Thislatter highly scattering structure clearly definesa NW-SE elongated shape in the Gulf ofPozzuoli and coincides with the western 1982-1984 seismogenetic volume (Orsi et al., 1999;D’Auria et al., 2011).

Petro-physics

Strategy and MethodsWe focus on the Mofete 1, Mofete 5, San Vito

1 and Licola 1 wells, according to the structuralposition defined before: i.e., along the structuralboundary, within the depression and outside ofthe caldera. We exclude from our analysis therocks from the San Vito 3 well, which show botha comparable position and a transitionalcharacter, in term of stratigraphy and mineralogy,to the Mofete wells (Rosi et al., 1983; De Vivoet al; 1989; see also discussion and conclusions).

Core samples taken from variable depths in theMofete 1, Mofete 5, San Vito 1 and Licola 1wells, given directly by Agip were analyzed.Conventional 30 µm- thin sections of the selectedsamples were prepared and studied by using apetrographic microscope ZEISS Axioscop 40equipped with reflection light (Istituto Nazionaledi Geofisica e Vulcanologia, Napoli). Primaryand, secondary (hydrothermal) mineral phaseswere identified and/or confirmed by using theSEM-EDS JEOL JSM 5310 electron microscopyinstalled at the Centro Interdipartimentale peranalisi geomineralogiche (CISAG) – University“Federico II” of Napoli, Italy, equipped withenergy dispersive system and operating at 15kV.Most useful for petrologic application was theacquisition of images from back scattered(BSEM) to establish the textural features and theminerals relationships. Clay minerals types arevery difficult to determine, due the small crystaldimension and variability; therefore, their depth-distribution was also extrapolated from publisheddata and not directly used in our analysis. Smallgrains, particularly when associated, pyrrhotiteand pyrite may confused under the microscopeand their distribution with depth requires X-raydiffractometric analyses.

ResultsOur results are consistent with those already

published in the literature showing a well-

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Secondary hydrothermal mineralsin buried rock... 395

developed and depth-dependent mineralalteration zoning related to the increasing-temperature of the hydrothermal system.However, our investigations further detail themineralogical assemblage of core samples,evidencing some accessory minerals, andtextural fabric of rocks not previously reported.Figure 3 shows the observed distribution ofhydrothermal minerals in the San Vito 1,

Mofete1, Mofete 5 and Licola 1 boreholes. Thisfigure also reports selected physical parametersfrom seismological investigations (Battaglia etal., 2008) and laboratory measurements (De Vivoet al., 1989; Zamora et al., 1994). A detaileddescription of the obtained results is reportedbelow for the three areas selected for this studyincluding the San Vito 1, Mofete 1, Mofete 5 andLicola 1 deep drillings.


Figure 5. (a) Calcite grown in feldspar fractures. Back-scattered electron image (sample SV1 1713); (b)Amphibole and brown biotite flakes in the bulk matrix (sample SV3 2864); (c) Chlorite plates crystallized infractures (sample MF1 402); (d) Rounded leucite grains diffused in the groundmass (sample MF1 1044); (e)Authigenic chlorite developed as plates in the cavities (sample MF1 1044); (f) Primary alkali feldspar withapatite and fluorite. Back-scattered electron image (sample MF1 617); (g) Calcite grown within rocks fractures(sample MF1 1714); (h) Gypsum crystals intergrown with primary alkali feldspar (sample MF5 2489); (i) Quartzgrain associated with muscovite and rutile relict. Back-scattered electron image (sample LC1 2634).



Figure 6. Horizontal and vertical section (top right) of the P-wave velocity model (left panels) and Vp/Vs ratio(right panels) modified from Battaglia et al. (2008). The wells described in the article are represented by theblack crosses. The black line shown in the second right panel shows the location of the vertical section showedabove. Black dots and green triangles indicate the earthquakes and stations, respectively, used for the tomography.



San Vito 1 wellSelected samples, listed according their depth

from bottom to the top of the San Vito 1borehole, have revealed the following features:at depths < 2130 m, rocks are represented byaltered volcanic tuffs and volcaniclastic tuffiteswith a reddish or greenish ashy-to-sandy matrix.In this matrix we recognized plates ofchloritized, elongate pumice fragments, variablycrystallized, vesicular scoriae and poorly-vesicular lithic clasts, as well as loose crystals.The fine matrix is strongly altered to by clayminerals, very difficult to define. In someinstances alteration is possibly represented byhematite or limonite, as testified by the brown-reddish colour in reflected light of someprismatic-fibrous minerals. Isolated brokenplagioclase and green pyroxene are also present,together with biotite and magnetite in themicrolite groundmass. Quartz and calcite arewidespread, commonly filling voids. Pyriteoccurs as isolated microcrysts and locally asaggregates in rock voids. At the shallowest depth(804 m), quartz (as tridimite) and zeolites wererecognised also within rock voids. Otherneogenic minerals that occur at shallower depthsare: feldspar, hematite, adularia, clinopyroxene,dolomite, titanite, spinel, epidote. They occurfrom below 2130 m. Epidote crystals areobserved in association with calcite at 1713 m(Figure 5a). EDS analyses also indicate thepresence of rodocrosite and garnet.

Rocks intersected at greater depths, becamemore massive and micro-crystalline, with ahomogenous texture and structure. At 2683 m therock shows a fluidal-intersertal groundmassmade of fine microlites mainly represented byplagioclase, alkali feldspar and biotitephenocrysts. Euhedral feldspars are generallyobserved in clusters (glomeroporphyritictexture), associated with quartz grain and acrystal with high interference colours, probablyepidote. The opaques are represented by pyrite,pyirrhotite and magnetite mostly in the veins. At

2862 meters, lithologies appear to be re-crystallized or a hydrothermally alteredsedimentary deposit. It consists of well-sortedgrains of quartz and feldspars that mostlyconcentrate within voids. Chlorite is widespreadand occurs in association with feldspar,pyroxene, epidote, hematite, sulphides. At depthsof about 2864 m, rocks are either effusive orsubvolcanic and characterized by elongatedbrown and squared black crystals in a whitegroundmass. Crystals have an isotropic and sub-idiomorphic structure and consist of nearlyequigranular feldspar grains in the bulk mass,biotite (up to 15%), amphibole (like Fe-actinolite, up to 10%; Figure 5b), opaques (< 10%) and rare pyroxene. No glass has beenidentified. Feldspar is generally euhedral tosubeuhedral plagioclase, includes abundantinclusions. Biotite occurs either as yellow-brownelongated and as tabular reddish-brown grains.Opaques minerals are mostly piryte and/orpyrrhotine and magnetite.

Mofete 1 and 5 wellsIn the first 250 m, the Mofete 1 well

intersected for yellow tuffs. Between 402 and617 m, we have identified fossils-bearingvolcanoclastic rocks. In particular, under theoptical microscope these rocks are characterizedby loose crystals reaching up to several mm insize and including plagioclase, pyroxene, biotite,rare magnetite, as well as fragments of pumice,scoria and lava. Pumices, scoriae and lavasfragments show variable degree of alteration, butin places maintaining their primary glassy andvesicular texture. The secondary paragenesis ofthese shallow pyroclastic deposits consistsmainly of chlorite within fractures (Figure 5c)and calcite. Sulphides are generally rare (< 5%by area). Based on fossils evidence, thevolcaniclastic tuffs were deposited between 79and 200 m b.s.l. At 1044 metres, the cored rockconsists of a mixture of greenish trachy-tephritelava and tuff fragments with many Sr and Ba




rich- leucite phenocrysts altered to analcyme(Figure 5d); the bulk matrix also includesplagioclase and pyroxene. Calcite and chloriteplats partially fill the different vacuoles (Figure5e). Locally there are minute (< 10 µm in size)zircon, epidote and Cu-Fe sulphide grains asrelict minerals. These mineral phases also occurin the rocks cored at 617 m where EDS analysesreveal the presence of titanite, quartz, epidote(allanite), fluorite apatite and albite (Figure 5f).At 1603 m the cored rock is a sandstone withintercalated siltstone laminae, showing quartzand feldspar grains as the main phenoclasticcomponents. In its matrix, fibrosis crystalsascribable to the sericite group (muscovite) arediffuse. Pyrite is present in small amounts and isrepresented by minutes cubic crystals.Sometimes small euhedral horneblenda crystalsappear. The matrix has a crystallized calcite vein,cutting through the entire core sample.

In the Mofete 5, tufaceous rocks occur at 1714m. They are inhomogeneous and consist of analtered ash matrix with feldspars and calcite; thelatter is the dominant mineral phase withinfractures (Figure 5g), together with rare adulariaand quartz. A crystalline aggregate consists ofquartz and calcite, with the latter being moredominant. In places, within voids, a radialaggregate of a greenish mineral is probablychlorite. Pyrite is the dominant opaque mineral (> 5%). Between 2222 and 2225.8 m homogenouslatite-trachitic lavas with a fluidal texture wererecognized. We also identified feldspar anddiopside as primary crystals. Calcite and chloritedominate the neogenic (alteration) assemblage.Sulphides and apatite are also common, whereasother phases include quartz, sericite, titanite,magnetite, dolomite, epidote, garnet and adularia.The sample cored at 2489 m shows a re-crystallized microgranular groundmass thatconsists predominantly of quartz, alkali feldspars(adularia) and gypsum (Figure 5h). The pyroxenesare subeuhedral to anhedral and generally alteredin the central part of the crystals. No calcite has

been detected. Pyrite with corrosion margins,gypsum, and REE-rich phosphates also occur. At2695 m an intrusive igneous or re-recrystallizedrock has been recovered. Based on EDSinvestigations, the majority of phenocrystscomprises sodalite and/or scapolite associatedwith smaller crystals of clinopyroxene and titanite.Quartz is also present. Pyrite/pyrrhotite is presentin large crystals with rounded margins (5-10%).

Licola 1 wellAt a depth of 2634 m in the Licola 1 drillhole,

arenaceous rocks with dark grey silt-to-sandstoneinterbeds is located between two lava flows.Under the microscope, these clastic rocks showsimilar secondary minerals namely: quartz andalkali-feldspars as the dominant phases, calciteplates (up to 5-10%), pyrite (< 5%) andwidespread fibrous crystals (probably muscovite).EDS-BSEM investigations allowed us also toidentify small (< 10 µm) crystals of rutile, zircon,titanite, Cu-Zn-Fe sulphides, in the partcharacterized by larger-sized crystals, and ofapatite, ilmenite, zircon, K-feldspar, and biotite,in the finer matrix (Figure 5i). The two lava rocksare massive, strongly altered, porphyritic, uniformin texture and in primary and secondaryparagenesis. The lava intersected at 2476 mconsists of acicular feldspar arranged in radial andspherulitic textures. Calcite is widespread asminute grains and as cement in the rock voids.Locally we identified small quartz and opaqueminerals; the latter are very few (< 1%) and arerepresented in order of abundance bypyrite/pyrrhotite and hematite. At 2637 metres, thelava rock has an intersertal-to-holocrystallinetexture and is, possibly, a trachytic-latitic sub-volcanic intrusion. The phenocrysts are mainlyplagioclase and alkali feldspar accompanied byminor pyroxene, with advanced sericite-typealteration. Other common secondary mineralsinclude pseudo-rhombohedral crystals of calciteand chlorite. The latter occurs as small acicularcrystals arranged in spherulitic clusters around the



feldspars and calcite. Quartz occurs in associationwith calcite; with lesser garnet, apatite andscapolite. Sulphides and oxides occurs asexclusively small and disseminated grains ofpyrite/pyrrhotite and magnetite/ ilmenite/titanite.

Discussion

The type and intensity of rock alteration andthe distribution of secondary mineral phases atthe Campi Flegrei caldera strongly depends onthe depth of the cored rocks. Figure 3 shows thecoherent variation with depth of bothhydrothermal alteration and temperatureincrement. Low temperature minerals (argillicfacies), such as zeolites and clay minerals, arein the upper sector of the investigated wells,gradually replaced by moderate temperatureminerals, like quartz, calcite and pyrite, which,in turn, deeper in the wells give way to higher-temperature mineral assemblages, like epidote,actinolite and garnet (epidote-actinolite zone).The zeolitizzation process was detected at adepth range 90-800 m and, as shown by De Vivoet al. (1989); namely, 100-800 m in the San Vito1 and Mofete 5 wells, respectively. The mostfrequent zeolite phases are phillipsite andchabazite, that generally characterize the weldedoutcropping tuffs, as reported by de Gennaro etal. (2000).

The temperature of zeolites crystallization is < 200 °C, as generally found in hydrothermalsystems (Browne, 1978; Caprarelli, 1997;Schott, 2001; Gebreivot, 2010). Thistemperature is in agreement with thosesuggested by presence of clay minerals (Steiner,1968; Kristmannsdottir, 1975; Elders et al.,1984; Harvey and Browne, 1991; Sener andGevrek, 2000). These temperatures alsocoincide with the homogenization temperaturemeasured on fluids inclusions by De Vivo et al.(1989). In particular, smectite is stable at 140°C, and illite, above 220 °C in active systems(Browne, 1984). Chlorite forming

microcrystalline aggregates or plats at the baseof the profile (Figure 3), suggests temperaturesof 220 °C (Kristmannsdottir, 1975). The argillicfacies at greater depth, is replaced by mineralsas epidote and calcite. Common minor phasesare quartz, chlorite, alkali feldspar and albite.The top of this zone is also marked by asignificant increase in pyrite, as tiny cubiccrystals grown in fractures, vesicles and veinsand widespread in the groundmass, in closeassociation with quartz and calcite. Pyrite formsabove 100 °C, is abundant in permeable zones(Reyes, 1990) and is a good indicator foraquifers (Reyes, 1990; Valentino and Stanzione,2004). In our samples pyrite occurs from aboutthe top to the bottom in San Vito 1 and Mofete1 and 5 wells, so that the range of thetemperatures is in perfect agreement withobservations made on other geothermal systems(Browne and Ellis, 1970; Browne, 1978). Atdepths greater than 1000 m, we identifiedanother highest-temperature mineralogicalassemblage represented by epidote, garnet andactinolite. The first epidote occurrence wasfound at 617 m depth (Mofete 1) at temperatureof the 250 °C (De Vivo et al., 1989), althoughmore commonly also occurs at deeper levels, ≥1044 and 2130 m in Mofete 1 and San Vito 1respectively, where the temperature wasestimated at around 300-400 °C (De Vivo et al.,1989). Epidote is usually stable in many activegeothermal system at temperatures above 200°C (Browne and Ellis, 1970; Eldes et al., 1979;Cavarretta et al., 1982; Bird et al., 1984), whereit generally replaces sodic plagioclase andclinopyroxene (Browne, 1978). Garnet andepidote grains are considered as indicator ofhigh-temperature conditions, over 300 °C (Birdet al., 1984). Based on thin-section studies, it isfound in rocks drilled at 1044 m in San Vito 1and at 2222.5 m in Mofete 5. Garnet is usuallynot very common in the wells, as it is presentonly above 320 °C (Browne, 1978). In oursamples, it is found as anhedral to subeuhedral




crystals infilling vesicles together with quartzand epidote. Fluid inclusion homogenizationtemperatures indicated values ranging from 300to 400 °C (De Vivo, 1989), in agreement withBrowne (1978). The detected mineralogicalassociation suggests hydrothermal temperatureshigher in the Mofete area than in the San Vito 1well. However, considering temperaturemeasured in the wells and following De Vivo etal. (1989), the Mofete data record a “fossil”hydrothermal mineral assemblage suggestive ofcooling of the system. Instead, the San Vito 1area maintains equilibrium between the actualand the fossil system with temperatures ofapproximately 400 °C. Therefore, we canhypothesize a stable geothermal system withtime in the San Vito 1 area. The San Vito 1 well,unlike at Mofete and Licola 1, typically showsa progressive disappearance of calcite.Moreover, here the garnet appears at a shallowerdepth with respect to the Mofete wells.

The degree of hydrothermal alteration withdepth is related to rock density increase andcorresponding porosity decrease (Figure 3). Thevariation of such features derives from theinfilling rock pores by neogenic hydrothermalminerals, quite apart from rock compaction dueto burial. Actually, the variable hydrothermalmineral assemblage, with the exception ofquartz, is characterized by a density ≥ 2.5 g/cm3

(Carmichael, 1982; Christensen, 1996), asmeasurements performed on cored samples (DeVivo et al., 1989; Zamora et al., 1994). Inparticular, the most widespread authigenicmineral is calcite (~ 2.71 g/cm3), whereasopaques (> 5 g/cm3) are the heaviest, hencedenser. We calculated that a tuff may increaseits density by about 0.3 g/cm3 due to an amountof 25% of calcite grown in vesicles, fracturesand/or been disseminated through the sample. Asimilar density increment is determined bywidespread (10% by area) opaques in thesamples. Finally, the re-crystallization of rocksinduced by thermal-metamorphic processes

causes the nearly complete loss of voids and theappearance of high density minerals typical ofhigher temperature, producing the highestdensity values (~ 2.5 g/cm3) of the deeper drilledrocks. The associated porosity decreases downto ~ 5%; with only some pyroclastics rocksshowing higher values, up to 10-15%. Overallrock-physics patterns are comparable with thoseof sedimentary basins (Magara, 1980; Maxwell,1964) and are in agreement with a volcaniccaldera depression, filled by dominantpyroclastic deposits and affected byhydrothermal fluids circulation inducingchanges on the rock properties.

Mineralogical features and physicalparameters are compared with Vp and Vp/Vsvalues from laboratory determinations (Figure3); in the last column the latter elasticparameters are also compared with thosederiving from in-situ seismological experiments.Besides, the high resolution tomographic resultsdo not resolve the entire caldera structure, withMofete and San Vito areas lying close to thelimit of data acquisition, whereas Licola is outof the resolved volume. Consequently some datawere just extrapolated using the velocitystratification model employed for hypocenterslocation (Battaglia et al., 2008; Judenherc andZollo, 2005). Furthermore, as stated before, thetomography resolution is > 500 m. Therefore,the comparison (Figure 3) is among differentrock volumes, with tomographic data averagedover several hundred meters of traversed rocks(Battaglia et al., 2008; Tramelli et al., 2006; DeSiena et al., 2010), which are much larger thatthe decimeter-sized samples measured inlaboratory. This problem is overcome by usingscattering and attenuation data that allowdetection of rock heterogeneities.

The neogenic (hydrothermal) minerals, aswell as the high-density and re-crystallizedrocks, show waves velocities higher than thosemeasured on the outcropping unaltered rocks.The P-wave velocities profiles, obtained at room




pressure and temperature on the cored rocks, fitquite well those derived by active and passiveseismology (Figures 3 and 6). Otherwise, theprofiles of the Vp/Vs ratios, strictly related tolithology (Wang 2000, and references therein),measured on core samples and those derivedfrom seismology, overlap only in the case of theSan Vito 1 well (triangles versus dotted greenline in Figure 3). In contrast, samples deeperthan ~ 2000 m of the Mofete wells, show Vp/Vsratios higher than those derived by seismology.They are also higher that those of the San Vito1 well. This feature relates to Vs decrease in theburied deposits and likely suggest the presenceof the water in rock voids. It is well established,that fluids do not sustain shear stress anddecrease the Vs without any Vp variation;otherwise, the presence of gas-bearing rocksgenerally changes the rock compressibility witha Vp decrease (e.g., Vanorio et al., 2002).

The different behaviour as revealed by multi-parametric profiles of the structurally differentSan Vito 1 with respect to the Mofete area, canbe interpreted in terms of volcanological andstructural features. The significant differentvolumes of emitted magma and the higherpercentage of active vents in the central-easternwith respect to the western (> 4 km3 vs < 1 km3,76 vs 24%, respectively) sector, suggest that thecentral-eastern part of the caldera was thepreferred pathway for magma ascent toward thesurface. This hypothesis agrees with data onmagma compositions that are more evolved inthe western. As a whole, we infer more efficientmagma fractionation processes in the westernsector of the caldera possible due to longerresidence time for the magmas at shallow depth.The occurrence of magmatic eruptions and theextrusion of relatively least-evolved magmas inthe eastern part of the caldera could be favouredby the extensional setting as detected by satellitedata (Vilardo et al., 2010). Furthermore, thefossil geothermal system and its cooling in thewestern sector, evidenced by disequilibria

between the detected and mineralogically-derived temperatures, can be explained by minorvolcanism and weak hydrothermal activity.

Conclusions

Fluid-rock interaction at the Campi Flegreicaldera has produced a suite of hydrothermalminerals that reflect the behavior of thereservoir(s) and, in turn, affect the elasticbehavior of buried rocks. The distribution of thehydrothermal minerals with depths is alsoindicative of past variations of the fluidschemistry. In agreement with De Vivo et al.(1989), the temperature of the geothermalsystem decreased at the Mofete (as well at theSan Vito 3) wells with time. The San Vito 1 wellis, instead, characterized by high-temperaturestability, as suggested by equilibrium betweenthe secondary mineralogical assemblage and thedetected on-site conditions. Furthermore, theVp/Vs depth-profiles highlight the differentbehaviors of the medium beneath the Mofeteand San Vito 1 areas. The San Vito 1 well is,interestingly, located within the area wheremagmas vented to the surface, at least in the last8 ka. It is also the closest to the Solfatara crater,where more than 1500 tonnes/day of CO2 arefluxed (Chiodini et al., 2001), and to thePozzuoli-Solfatara area, characterized byintense volcano-tectonics (Di Vito et al., 1999),maximum ground deformation and detectable inpresent-day extension (Orsi et al., 1996; 1999;Vilardo et al., 2010). The scattering and velocitytomography (Judenherc and Zollo, 2005;Battaglia et al., 2008) confirmed the presenceof a highly fractured medium below Solfatara.As a whole, the different subsurface conditionsevidenced for the San Vito 1 area with respectto the Mofete (and San Vito 3) depends, in ouropinion, on the volcanic system architecture andbehaviour. In particular, the central-eastern partof the caldera is the preferential pathway forboth the gas escape and fluid circulation (Caliro




et al., 2007) as well as magma ascent to thesurface.

Acknowledgments

We are grateful to Prof. Paola Petrosino (“FedericoII” University, Napoli, Italy) for allowing access to thelaboratory of electron microscopy. We acknowledgethe technician dott. Roberto de Gennaro for skillfulassistance in SEM observations and EDS analyses atthe “Centro Interdipartimentale per analisigeomineralogiche (CISAG) - “Federico II” University,Napoli, Italy. We are grateful to Dott. GiuseppinaBalassone for useful discussions and suggestions.Prof. Franco Pirajno and an anonymous reviewer arethanked for their revision. The Authors wish to expresstheir gratitude to the Editor Prof. Antonio Gianfagnafor the opportunity in contributing to this specialvolume. This work has been supported by EU-VIIFPGEISER project (ENERGY.2009.2.4.1).

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Submitted, July 2011 - Accepted, November 2011



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Secondary hydrothermal minerals in buried rocks at the ...periodicodimineralogia.it/2011_80_3/2011PM0027.pdfSecondary hydrothermal minerals in buried rocks at the Campi Flegrei caldera,